There is a desire for an illuminator which enables a reflection type liquid-crystal display to be read in the dark or the like. The present inventors attempted to apply a backlight for use in transmission type liquid-crystal displays to a front light system in which the backlight is disposed on the viewing side of a liquid-crystal cell. This backlight has a light conductive plate in which incident light from a side edge is emitted through a light-emitting means from one of the upper and lower surfaces. In the front light system, the displayed information is viewed through the light conductive plate.
However, the backlights employing conventional light conductive plates have been difficult to put to practical use because they pose problems that the display is insufficient in contrast in an illuminated state, has insufficient brightness in an unilluminated state, and has display disturbance, etc. For example, a liquid-crystal display having a light conductive plate employing diffusing dots or fine roughness as a light-emitting means has the following problems. The display is insufficient in contrast when viewed in an illustrated state. When viewed in an unilluminated state with external light, e.g., indoor lighting, the display is insufficient in contrast and has insufficient display brightness. In addition, the displayed images on the unilluminated display, which are viewed through the light conductive plate, are considerably disturbed and have poor clearness.
On the other hand, a light conductive plate has been proposed which employs as a light-emitting means a staircase-like prismatic structure having inclined faces with an angle of inclination of 45 degrees and flat faces with an angle of inclination of 0 degree (see JP-A-62-73206; the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Even a liquid-crystal display having this light conductive plate has a problem that it is insufficient in contrast and display brightness when viewed in an illuminated state.
The above liquid-crystal display further had the following problem. Part of incident light is reflected by the lower surface of the light conductive plate without entering the liquid-crystal cell, etc., and leaks through the upper surface thereof. When this liquid-crystal display is viewed from the upper side, the resultant leakage light is superposed on displayed images, regardless of whether the display is in an illuminated state or not, to cause white blurring and reduced contrast, resulting in impaired perceptibility.